Dissimilar metals pose the risk of "galvanic corrosion". Some more so than others. Throw a copper washer in the bottom of an aluminium tinnie for a couple of months and see what happens.
( No, don't do this )The problem is exacerbated when an electrolytic medium is added to the mix, e.g moisture.
I've had a hole "eaten" through the side of an aluminium tipping trailer from a spot of copper concentrate that was missed and not cleaned out for a couple of months. Working in an underground copper mine really shows you how corrosive copper can be. We had to replace front ends of underground "boggers" that were unservicable due to corrosion eating through 60mm thick chassis structure. You'd cry if you saw the rust in a 3 year old Toyota chassis, we scrapped plenty of those too.
Yes, stainless rifle barrels ( or chrome moly ) should be more resistant than an alloy trailer/tinnie but the potential is still there. You still have two dissimilar metals. I've cleaned up plenty of second hand barrels to discover enough pitting under the copper to realize the potential risk.
In my experience, copper fouling is increased with higher velocities, longer bearing surfaces, high temperatures,"rough" bores and pure volume of shooting.
Best to get it out.
Sweets 7.62 is my go to, applied with a nylon brush, patched out after 10 -15 minutes. Repeat until the blue stops coming out on the patches. Then use a powder solvent on a bronze brush, patch clean and repeat the whole process until both solvents produce clean patches.
Due diligence must be applied when cleaning and the use of quality, correct size rods, bore guides, brushes and jags.( More good barrels have been stuffed by sloppy cleaning practices than anything else. )
Some might say this is over the top but I apply this method to my competition rifles and they give very good barrel life and accuracy. Now all I need to do is shoot better hey
Strikey.